1. Field of the Invention
The present invention relates to a coated cutting tool member that resists chipping and wear for long periods of time during cutting operations.
2. Description of the Related Art
Coated carbide cutting tool members are preferably composed of a tungsten carbide-based cemented carbide substrate and a hard coating layer preferably made of aluminum oxide (hereinafter referred to as "Al.sub.2 O.sub.3 "). Preferably, they further comprise a cubic-type titanium compound layer preferably including at least one layer of titanium compound having a "cubic" crystal structure preferably selected from titanium carbide (TiC), titanium nitride (TiN), titanium carbonitride (TiCN), titanium carboxide (TiCO), titanium nitroxide (TiNO) and titanium carbonitroxide (TiCNO). The hard coating layer is formed preferably by means of chemical vapor deposition and/or physical vapor deposition and have an average thickness of 3 to 20 .mu.m. X-ray diffraction can confirm that the crystal structure of a titanium compound layer is cubic-type (hereinafter referred to as "cubic-type titanium compound layer"). A coated carbide cutting tool member having a hard coating layer, wherein the first layer is TiN, the second layer is TiCN, the third layer is TiCNO, the fourth layer is Al.sub.2 O.sub.3 and fifth layer is TiN disclosed in Japanese Unexamined Patent Publication No.7-328810. These coated carbide cutting tool members are widely used in various fields of cutting operations, for example, continuous and interrupted cutting operation of metal work pieces.
It is known that cubic-type titanium compound layers have granular crystal morphology and are used for many applications. Recently, a TiCN layer that has a longitudinal crystal morphology has found use as a highly wear resistant coating layer. TiC layers have been used as highly abrasion resistant materials in many applications. TiN layers have been used in many fields, for example, as an outermost layer of a coated cutting tool member and for various decorative products, because of its beautiful external view like gold. Layers of Al.sub.2 O.sub.3 have several different crystal polymorphs, among which the alpha-Al.sub.2 O.sub.3 is known as the thermodynamically most stable polymorph, having a corundum structure. Typically, an Al.sub.2 O.sub.3 coating formed by CVD has three kinds of Al.sub.2 O.sub.3 polymorphs, namely, stable alpha-Al.sub.2 O.sub.3, meta-stable kappa-Al.sub.2 O.sub.3 and amorphous Al.sub.2 O.sub.3.
In recent years, there has been an increasing demand for labor-saving, less time consuming cutting operations. These operations preferably include high speed cutting operations such as high speed feeding and/or high speed cutting. In these cutting operations, cutting tools are exposed to extraordinarily severe conditions. During these high speed cutting operations, the temperature of the cutting edge rises to 1000.degree. C., or more and work chips of exceedingly high temperature are in contact with the surface of the rake face of the cutting tool. This phenomenon accelerates the occurrence of crater wear on the rake face. Thus, the cutting tool is chipped or damaged at a relatively early stage.
In order to circumvent this situation, a coated carbide cutting tool which has a relatively thick Al.sub.2 O.sub.3 layer has been examined and produced. The Al.sub.2 O.sub.3 layer has favorable properties such as extremely high resistance against oxidation, chemical stability and high hardness which meet the demands of cutting tools that are used under high temperature conditions. However, applying Al.sub.2 O.sub.3 layers to cutting tools does not work out as one desires. Adhesion strength of the Al.sub.2 O.sub.3 layer to an adjacent cubic-type titanium compound layer is usually not adequate, especially when the Al.sub.2 O.sub.3 polymorph is alpha-type, and it is also inevitable that the Al.sub.2 O.sub.3 layer has local nonuniformity in its thickness when it becomes a thicker layer. The Al.sub.2 O.sub.3 layer tends to be thicker at the edge portion of the cutting tool, for example, than that at the other portions of the tool. When the thick Al.sub.2 O.sub.3 layer is applied as a constituent of a hard coating layer, it is likely to show relatively short life time, for example, due to an occurrence of some kind of damage such as chipping, flaking and breakage.
As the cutting speed of various cutting operations continue to increase, thicker coatings of Al.sub.2 O.sub.3 will be required to protect carbide cutting tools. With thicker Al.sub.2 O.sub.3 layers, tool-life time will be more sensitive to both the adhesion strength between Al.sub.2 O.sub.3 layer and cubic-type titanium compound layer as well as the toughness of Al.sub.2 O.sub.3 layer itself. Methods for adhering Al.sub.2 O.sub.3 layers to other compound layers and methods for making tough and thick Al.sub.2 O.sub.3 layers continue to grow in importance with increasing demand for cutting tools that work at higher and higher speeds.